The quality and resolution of an MRI image of a subject is sensitive to inhomogeneities in a static polarizing magnetic field that is used in the imaging process to polarize nuclei in the subject. The polarizing field is ideally uniform over the volume of the subject and deviations from a desired constant value can degrade the image.
Particularly susceptible to the effects of inhomogeneities in the polarizing field are MRI images of subjects produced using gradient echo (GE) imaging, water and fat separated imaging or echo planar (EPI) imaging. Often, inhomogeneities in the polarizing field are produced as a result of magnetic susceptibility of the material or tissue of a subject imaged. Inhomogeneities resulting from magnetic susceptibility of material imaged can be especially problematic in in-vivo imaging for which strong magnetic fields are generally required.
In order to compensate for distortions in an MRI image of a subject caused by inhomogeneities in the polarizing magnetic field, the inhomogeneities are measured as a function of position. The measured inhomogeneities are used to correct the image distortions mathematically and/or to determine currents for shimming coils used to produce shimming fields to moderate the inhomogeneities.
Measurements of the magnetic field inhomogeneities are often made by acquiring two sets, hereinafter referred to as "k-space scans", of data in k-space that characterize the subject in k-space. An MRI imaging sequence is used to acquire a first k-space scan of the subject at a first time. Subsequently, following an accurately determined delay time, the imaging sequence is repeated to acquire a second k-space scan of the subject at a second time. Each of the k-space scans is used to generate a spatial image of the subject. The delay time results in phase differences between values of the two generated spatial images, which phase differences are functions of magnetic field inhomogeneities. The phase difference at a given position is proportional to the delay time and the magnitude of an inhomogeneity in the magnetic field at the position. By dividing the phase difference at the position by the time delay, the inhomogeneity in the field at the position is determined.
The acquisition of data for two images using conventional magnetic field inhomogeneity measurement procedures generally requires a time period of a few seconds or tens of seconds. In an article entitled "In Vivo Rapid Magnetic Field Measurement and Shimming Using Single Scan Differential Phase Mapping" by Kanayama et al, in MRM 36:637-642, (1996), which is incorporated herein be reference, a procedure for measuring inhomogeneities in a polarizing magnetic field from two images comprising 128.times.128.times.16 pixels each is described. In the article a data acquisition time of over a hundred seconds is reported for acquiring the two images. An article entitled "Automated Shimming at 1.5 Tesla Using Echo-Planar Image Frequency Maps", by Reese et al, in JMRI 5:739-745, (1995), which is incorporated herein by reference, also describes measuring inhomogeneities using two images of a subject. As reported in this article, each image is generated from 25 images of slices of the subject and ten seconds are required to acquire data for the two images.
In many situations long data acquisition times for magnetic field measurements compromise the usefulness of the field measurements. For example, when imaging biological processes in human organs, changes often take place in less than a second and field inhomogeneities are a function of the magnetic susceptibility of tissues in the organs. For these situations, field measurement times on the order of seconds or more are often of limited usefulness.